The invention relates to a method for separating super-critical gas from substances dissolved in the gas.
The extraction of substances using supercritical carbon dioxide (called below scCO2) is a well-known method, which has important advantages particularly in that it does not require environmentally unacceptable organic solvents. The disadvantage however is high energy costs caused by the compression of the carbon dioxide after the separation of the extracted substances from the supercritical carbon dioxide. With the utilization of the dissolving capability of the scCO2 for the separation of substances, the substances dissolved in the supercritical gas are precipitated generally by a reduction of the pressure from the process pressure to almost critical pressure. The amount of supercritical gas required for the extraction is relatively high. Although such gas has a higher dissolving power than normal gases, the concentration of the dissolved substances is relatively small since many of the substances of interest have a low volatility.
In technical processes, the scCO2 must be kept in a closed flow circuit. The necessary compression procedures require a high energy input. As a result, the process of substance separation with supercritical gases is uneconomical when compared with conventional methods including distillation, vacuum-distillation, short-path distillation and molecular distillation.
It has often been tried therefore to separate the substances dissolved in the supercritical carbon dioxide by suitable procedures, without essential pressure reduction from the supercritical gas pressure. There are several suitable processes. With adsorption, the substances dissolved in the supercritical gas are adsorbed by an adsorption material such as activated carbon or silicon gel. Then, however, the adsorbed substances must be separated from the adsorption material which again may be quite complicated. With the removal of caffeine from green coffee beans by adsorption, an activated carbon for example, the difficulties were so great that the caffeine could not be recovered. This process has therefore been changed and absorption has been used in order to be able to recover the valuable caffeine without pressure reduction.
In the decaffeination process, the caffeine dissolved in the supercritical carbon dioxide is washed out of the supercritical carbon dioxide by water. In this case, such a procedure is advantageous because the caffeine is soluble in water. From the aqueous solution, the caffeine can then be recovered by crystallization.
However, most of the substances of interest for separation with supercritical gases are not soluble in water. Therefore, the process of absorbing in water is not an option, since it would have to be followed by the conventional additional separation step of distillation, which is what the extraction by supercritical gas is to avoid.
A separation-by way of membranes is therefore highly attractive. Substances dissolved (extracts) and supercritical gas (solvent) can be separated from each other without an essential reduction of the supercritical gas pressure.
It is therefore the object of the invention to provide a process for the separation of substances dissolved in supercritical gas without the need for a substantial reduction in the supercritical gas pressure.
In a method for the separation of supercritical gas from substances dissolved in the gas with a membrane through which the gas passes while the substances dissolved in the gas are retained, a pore-free membrane of a polymer perfluoro-2,2-dimethyl-1,3-dioxol is used as a membrane in the separation process.
Such a process will be described below on the basis of the accompanying drawings.